One state-of-the-art keratometer for determining the topography of the corneal surface (SU, A, 1,115,716) is known to comprise a source of light, measuring marks shaped as concentric reflecting rings and a radial grid, which are located on a spherical concave surface whose axis is aligned with the optic axis of a projection lens, wherein use is made of a telecentric path of light rays in the object space.
A cardinal disadvantage of the known keratometer resides in that the measuring marks are situated at a final distance from the corneal surface, which provides for but a little area of the cornea arrangement zone within which cornea displacement will not affect adversely the measurement accuracy. Besides, such a little area of the cornea arrangement prevents pinpoint-accuracy measurement of the surface of corneas featuring high degree of asphericity. Another disadvantage inherent in the construction in question is the use of large-diameter (over 150 mm) concentric rings as the measuring marks, since high-accuracy production of such rings comes across some technological difficulties, whereas production inaccuracies of the rings affect adversely the measurement accuracy.
Further disadvantages of the known keratometer reside in its large overall dimensions and mass. On the other hand, the necessity for accurate orientation of the corneal surface with respect to the instrument and accurate setting of the measuring marks involves further complication of the keratometer construction. In addition, a patient's head is located close to the instrument during the measuring procedure, thus presenting inconvenience to the patient. The disadvantages mentioned above make impossible an efficient use of the keratometer in the practice of medical studies and rules out completely a possibility of its application in carrying out microsurgical procedures on the eye.
One more keratometer (SU, A, 1,337,042) is known to comprise an illumination system which incorporates the following components located before the cornea: a light source, an objective lens in the form of a spherical lens, measuring marks interposed between the light source and the spherical lens and made as luminous holes in a diaphragm shaped as a concave sphere whose centre is aligned with the centre of the spherical lens, the principal sections of prisms with two reflecting faces being situated in the meridional planes of said spherical lens. The aforesaid prisms are so oriented that the beams of parallel light rays emerging from the spherical lens at different angles and reflected from said prisms, intersect in the zone of location of the cornea under examination, and the projection lens makes use of a telecentric path of light rays in the object space.
A substantial disadvantage from which the known keratometer suffers resides in discreteness of the measuring marks, which fails to provide a required accuracy of measurement of the surface of unsymmetrically shaped corneas, since the shape of an entire corneal surface is judged by the results of measurement of its separate portions. Besides, manufacture of a spherical lens and a great many prisms offers some technological difficulties, while installation of such prisms involves further sophistication of the construction of the keratometer in question, as well as increasing of its overall dimensions and mass. Moreover, the keratometer construction fails to provide convenience in carrying out medical studies since it requires that a patient's head be situated close to the instrument, which is inconvenient for the patient.